Modified naphthalene formaldehyde resins can be used for widespread applications such as an electrical insulating material, a resin for resist, a semiconductor sealing resin, an adhesive for printed wiring board, a matrix resin for electrical laminate or prepreg to be mounted in electrical instruments, electronic instruments, industrial instruments, etc., a buildup laminate material, a resin for fiber-reinforced plastic, a sealing resin for liquid crystal display panel, a paint, a variety of coating agents, an adhesive and the like.
An aromatic hydrocarbon resin obtained by allowing a polycyclic aromatic hydrocarbon composed mainly of a monomethylnaphthalene and/or a dimethylnaphthalene and paraformaldehyde to react with each other in the presence of an aromatic monosulfonic acid is known, and the obtained resin is excellent in compatibility with a liquid epoxy resin and solubility in xylene (see Patent Document 1).
Also, there is known a method for obtaining a phenol resin having a structure in which naphthalene and a phenolic hydroxyl group-containing compound are bonded via a methylene group through a reaction between a methoxymethylene naphthalene compound and a phenolic hydroxyl group-containing compound such as phenol, cresol, naphthol, etc. in the presence of diethyl sulfate (see Patent Document 2).
Now, in order to use a resin obtained through a condensation reaction of a polycyclic aromatic hydrocarbon formaldehyde resin and a phenolic hydroxyl group-containing compound for a thermosetting resin application, it is more preferable that the obtained resin is polyfunctional.
However, in the case where naphthalene or a monomethylnaphthalene is used as a raw material of resin, it is difficult to obtain a polyfunctional resin by a usual method so that it was necessary to perform a special reaction such as an interface reaction (see Patent Documents 2 and 3).    [Patent Document 1] JP-A-54-86593    [Patent Document 2] JP-A-2004-91550    [Patent Document 3] JP-A-61-228013    [Patent Document 4] JP-A-11-92543